1. Field of the Invention
The present invention relates to a photonic crystal optical device having an active layer with an optical waveguide formed by a photonic crystal.
2. Description of the Related Art
A photonic crystal has a periodic structure formed by arranging on a semiconductor or the like, mediums of a refractive index different from that of the semiconductor, in a period of about a wavelength of light. Application of the photonic crystal to various optical devices has been studied. A photonic band gap (PBG) to light is formed in the photonic crystal, according to a solution of the Maxwell equation in a periodic field. Light of a wavelength corresponding to the PBG cannot be propagated to any direction in the photonic crystal. When a properly designed defect is introduced into this periodic structure, light is localized in this defective part. Therefore, an optical resonator can be realized by introducing a point defect into the periodic structure, and an optical waveguide, that is, a photonic-crystal optical waveguide, can be realized by introducing a line defect into the periodic structure, for example (see Nonpatent Literatures 1 and 2).
FIG. 18 is a top plan view schematically depicting a conventional photonic crystal optical device used in an optical amplifier or a laser. FIG. 19 is a cross-sectional view cut along a line F-F of the photonic crystal optical device in FIG. 18. FIG. 20 is a cross-sectional view cut along a line G-G of the photonic crystal optical device in FIG. 18. The photonic crystal optical device 400 has an active area 402 and a passive area 401 integrated on a semiconductor substrate S4. The active area 402 has a photonic-crystal optical waveguide 404 formed by providing plural circular holes 408 periodically laid out in the main surface direction of an active layer 412 from an active-area upper cladding layer 413 to a position deeper than a lower surface of the active layer 412, excluding a line defective part which is to guide a light of a predetermined wavelength, in an active area growth portion in which the active layer 412 is grown as an active-area core layer between the active-area upper cladding layer 413 and an active-area lower cladding layer 414. The passive area 401 has a deep ridge passive optical waveguide 405 formed to be optically connected to the photonic-crystal optical waveguide 404, in a passive area growth portion in which a passive-area core layer 411 is grown between a passive-area upper cladding layer 413 and a passive-area lower cladding layer 414. A reference numeral 418 denotes a contact layer, 430 denotes an insulating film, 431 denotes an upper electrode, and 432 denotes a lower electrode. In FIG. 18, the insulating film 430 and the upper electrode 431 are expressed as transparent for the sake of explanation.
The photonic crystal optical device 400 can have a considerably long net optical-path length even with a short element length, by using a low group-velocity effect of the photonic crystal in the photonic-crystal optical waveguide 404. As a result, effective gain can be set considerably high. Therefore, the photonic crystal optical device 400 is small and operates with low power consumption. The low group-velocity effect is a phenomenon that the group velocity becomes zero by Bragg reflection near a boundary of the Brillouin Zone in the wave number space of a photonic crystal, that is, near a wave number value which is an integer times k=π/a where “a” represents a lattice constant of the photonic crystal. The zero group-velocity point is a point at which a differential of an angular frequency with respect to a wave number is zero on a dispersion curve showing a relationship between the wave number and the angular frequency. The photonic-crystal optical waveguide 404 can be designed to have a zero group-velocity point on the dispersion curve at either both or one of a high-frequency side and a low-frequency side within a transmission band. The active layer 412 is designed to have a gain at the zero group-velocity point and in a frequency band near this point on the dispersion curve of the photonic-crystal optical waveguide 404.
The photonic-crystal optical waveguide 404 is made by sequentially forming the active area 402 and the passive area 401 on the semiconductor substrate S4, and thereafter forming a predetermined pattern of the holes 408 using lithography and etching.
Nonpatent Literature 1: A. Talneau, et al., “High external efficiency in a monomode full-photonic-crystal laser under continuous wave electric injection”, Applied Physics Letters, v. 85, no. 11, pp. 1913 to 1915 (2004)
Nonpatent Literature 2: H. Takano, et al., “In-plane-type channel drop filter in a two-dimensional photonic crystal slab”, Applied Physics Letters, v. 84, no. 13, pp. 2226 to 2228 (2004)
However, according to the conventional photonic crystal optical device, there has been a problem in that power efficiency falls because of a light reflection occurring at a connection between the active area and the passive area due to a slight deviation of about a few micrometers in the patterning at the time of forming a pattern of holes, so that efficiency of optical connection between the active area and the passive area is deteriorated. On the other hand, to prevent the reduction in power efficiency, the deviation of the patterning needs to be prevented; however, it is difficult to control the deviation of about a few micrometers in the patterning.